Plunger Pull Students observe that air exerts pressure. Related Web Pages for Students • http://eo.ucar.edu/kids/sky/index.htm • http://eo.ucar.edu/webweather/ • http://www.ucar.edu/learn Directions 1. Put the plungers together, with concave sides facing one another. Student Learning 2. Notice that they touch but do not hold together. Objectives 3. Put the plungers firmly together, with concave sides facing one • Students learn that air exerts another again, put this time push the air out that is held and shared pressure at 14.7 pounds per between them. Notice what happens. column inch at sea level on 4. Hold the plungers by their outside ends and try to pull them apart. average (1 kg per sq. cm.) Do not twist or peel them, just pull. Are you able to pull them apart? • Students learn that a vacuum is an area of Ask yourself the following questions: empty space. 1. Is suction a force? If not, what causes it? 2. How much weight is exerted on the plungers from air pressure? How would you go about determining this quantity? Time • 10 minutes, activity; Background Information 10 minutes, discussion Although air is invisible, it still takes up space and has weight. In fact, it may surprise you to know that air weighs 14.7 pounds per column inch Materials at sea level, or put another way, over 2,100 pounds per column cubic For each pair of students: foot! Wow! The reason we don’t feel its weight is because air, like all • One set of suction cups fluids, doesn’t just push down. Instead, it pushes in all directions. or plungers, any size. Water has weight too, but you aren’t crushed when you swim to the bot- Wooden handles are best tom of a deep pool because water, like air, also pushes in all directions. to remove. But just try to lift all that water you’re swimming under. It weights over 62 pounds per cubic foot! National Standards • A: Science as Inquiry In 1654, Otto von Guerick performed a demonstration similar to this • D: Earth Science activity with the plungers. He used two metal hemispheres that were 22 inches in diameter and placed them together in the shape of a single sphere. He had invented the world’s first vacuum pump shortly before 1654, which pumped air out of his sphere instead of into it. When he did so, the two hemispheres held together tightly. No human could pull Otto’s sphere apart, so in front of Emperor Ferdinand III of Prussian Saxony (now Germany) and many others, he attached two eight-horse Otto von Guerick’s sphere is teams to each end of his sphere. Despite a great effort, the horses known around the world as could not pull his sphere apart. Afterall, they were trying to pull apart the Madgeburg Sphere nearly 3 tons! because it was built in Guerick’s home-town of When air is inside the sphere, it exerts the same amount of force as the Madgeburg in what today is air on the outside of the sphere. When you remove the air inside of the Germany. sphere, however, the air on the outside presses the two halves of the sphere together. If you peel the plungers apart slightly and let air back inside, the sphere’s two sides will no longer stick together! The force on the inside and outside will once again be the same. 4.